To begin with, a conventional information terminal is explained.
FIG. 22 is a configuration diagram of a conventional extension telephone system, and FIG. 23 is a frequency band diagram for audio signal of the conventional extension telephone system. The operation is explained below while referring to FIG. 22 and FIG. 23.
Hitherto, the information terminal having the extension function is classified into a main apparatus 1 and information terminals 2 and 3.
The flow of an audio signal for an outside call sent from the information terminal 2 is as follows: the audio signal entered from a microphone 11 is modulated by a modulator 9 into an audio signal frequency band 19 in FIG. 23, and is demodulated into an ordinary audio signal (telephone signal frequency band 18 in FIG. 23) by a demodulator 6 of the main apparatus 1 through a private telephone line 17. It is then sent to an exchange through a subscriber's line 60 by a network interface circuit 4 of the main apparatus 1.
To the contrary, the flow of an audio signal received at information terminal 2 is as follows: the audio signal sent from the exchange through the subscriber's line 60 is modulated into an audio signal frequency band 20 by the modulator 5 in the main apparatus 1, and is demodulated into an ordinary audio frequency band by the demodulator 10 in the information terminal 2 through the private telephone line 17, and is output from a speaker 12.
On the other hand, in the case of extension service between the information terminal 2 and information terminal 3, the flow of an audio signal sent from information terminal 2 is as follows: the audio signal entered from the microphone 11 is modulated into an audio signal frequency band 19 by the modulator 9, and is demodulated into a telephone signal frequency band 18 in a demodulator 14 of the information terminal 3 through the private telephone line 17, and is output from a speaker 16 of the information terminal 3.
To the contrary, the flow of an audio signal received at information terminal 2 from information terminal 3 is as follows: the audio signal entered from a microphone 15 is modulated into an audio signal frequency band 20 by a modulator 13, and is demodulated into an ordinary audio frequency band 18 by the demodulator 10 in the information terminal 2 through the private telephone line 17, and is output from the speaker 12 of the information terminal 2.
Thus, in the case of an outside call from the information terminal, the audio signal frequency bands 19, 20 in FIG. 23 were used, and also in the case of extension service, the same audio signal frequency bands 19, 20 were used.
However, since the audio signal frequency bands 19, 20 are used both in an outside call from the information terminal and in extension service, if there are two service bands only, that is, audio signal frequency bands 19 and 20 only, while one information terminal is busy with outside call, extension service is not possible in other terminal.